The present invention pertains to polarized lenses, and in particular to polarized high-impact-polymer lenses manufactured by an injection/coining injection molding process.
Polarized lenses for eyewear have been in use for over 50 years (see, e.g., U.S. Pat. No. 2,237,567 to Land, and U.S. Pat. No. 2,445,555 to Binda). Polarized lenses can selectively eliminate glare that originates from the reflection and subsequent polarization of light from flat surfaces such a pavement, water, sand or snow. Thus, polarized lenses are particularly useful for outdoor activities such as driving, fishing, sailing, sunbathing, and skiing.
One popular type of polarized lens is formed from a sheet polarizer, which is a thin layer of polyvinyl alcohol sandwiched between two layers of a cellulosic film, such as cellulose acetobutyrate or cellulose triacetate. Although sheet polarizer lenses are light-weight and inexpensive to produce, they deform easily, are not highly impact resistant, and have no corrective power (i.e., are plano).
Many popular and economical polarized lenses are based on iodine. Others are based on dichroic dyes. Iodine polarizers have high polarizing efficiency. Dichroic dyes are typically less efficient but have higher temperature stability and higher moisture resistance than iodine-based polarizers.
Early improvements in the lenses involved placing a wafer of sheet polarizer inside of a mold, and casting CR39 monomer around the wafer. U.S. Pat. No. 4,090,830 to LaLiberte describes this casting process. The mold is then placed in a water bath and cured at varying temperature for 12 to 24 hours during which time the monomer polymerizes into a hard precisely curved shape. An improvement to this process is described in U.S. Pat. No. 3,940,304 to Schuler. The improvement involves coating a polarizing wafer with a thin tie coat of melamine formaldehyde, then no forming it to match the curvature of one of the mold surfaces, and placing it inside of the mold before it is filled with CR39 monomer. The resulting lens is polarized and can be either plano, or a prescription lens with power. CR39 lenses are hard but do not have high impact resistance. They are suitable for dress eyewear, but not for sport applications or for children who engage in rough and tumble play.
Another process of forming a polarized lens is described in U.S. Pat. No. 6,328,446 to Bhalakia et al. and involves laminating the polarizing wafer onto the front of an existing lens. However it has been frequently found that the lamination process is difficult and results in a low yield. A common problem encountered by this laminating approach is that variations in the thickness of the single layer of adhesive can lead to distortion. Moreover, laminations are particularly difficult with lenses having different curvatures in different parts of the surface, such as occurs in bifocal or progressive powered lenses.
Another process for forming a polarized lens is described in U.S. Pat. No. 6,334,681 to Perrott et al. and in U.S. Pat. No. 6,256,15 Coldray et al. The process involves laminating the polarizing wafer between two optical members. This approach is more costly than that involving a single optical member, though it has the advantage of the polarizer being well protected between the optical members. Optical distortion, caused by variations in the curvature of the polarizing wafer, are canceled out by variations in the thickness of the adhesive, provided that the index refraction of the polarizer matches that of the adhesive.
More recently, U.S. Pat. No. 5,051,309 to Kawaki et al. discloses a polarized lens in which the polarizing layer is sandwiched between two sheets of polycarbonate. The polycarbonate is stretched, resulting in high stress and thus high birefringence. The stretch axis is aligned with the absorption axis of the polarizer. The birefringence of the polycarbonate is not noticed when viewing the lens perpendicular to its surface. However when viewed at an angle to its surface, the high birefringence causes interference fringes. By using highly stretched polycarbonate, the fringes are high order and washed out and so are not noticeable by the user of the lens. The resulting sandwich of polycarbonate is then thermoformed. Polycarbonate requires higher temperatures and longer times to thermoform than do cellulosic films. The polarizer is preferably a dichroic polarizer, although some iodine may be included to improve the polarizing efficiency. Unfortunately, polycarbonate has relatively high optical dispersion, which results in chromatic dispersion. Thus, when certain objects such as streetlights are viewed off-axis, a halo of blue light is seen to one side of the image.
Although polycarbonate is known for high impact resistance, its strength is reduced by internal stresses. Thus, to meet impact tests for safety glasses, certain polycarbonate lenses are made 2.4 mm thick. However, when such lenses are mounted in eyeglass frames with a wraparound design, they have residual power and prismatic effects. The lenses often do not meet the European Class 1 standards and fall into the Class 2 category. This characterization of the lenses as xe2x80x9csecond classxe2x80x9d is a drawback.
One approach to forming lenses from polycarbonate is to use thermoformed polycarbonate sheet as inserts and then injection molding polycarbonate around the sheet. Since each side of the injection mold can be precisely made, the resulting lens has no unwanted power or prismatic effect. By correctly designing the mold surfaces, prescription ophthalmic lenses of any desired power can be manufactured. The bond between the polycarbonate polarizer and the injected polycarbonate is quite strong. However, conventional injection molding introduces considerable stress into molded parts. This stress adds to the stress in the thermoformed polycarbonate insert. Great care must be taken in mounting these lenses in frames so that the lens fits the frame groove exactly. Otherwise, additional stress is introduced by the frame, which can cause crazing of the edges and birefringent stress patterns when the lens is viewed off-axis.
Many attempts have been made to injection mold polycarbonate or polymethylacrylic around wafers containing Iodine-based polarizers. Unfortunately, all previous attempts have failed because the polarizer was destroyed by the high temperature necessary to achieve flow of the injected polymers.
A first aspect of the invention is an article comprising a polarizer sandwiched between first and second layers of cellulosic material. The polarizer plus the layers of cellulosic material form a polarizing insert. The polarizing insert is formed (e.g., thermoformed) to have a curvature corresponding to the surface curvature of a plate of an injection mold assembly. A high-impact polymer is formed adjacent at least one side of the polarizing insert by an injection/coining process of injection molding that results in minimal stress in the polarizing insert and the high-impact polymer.
A second aspect of the invention is a polarized lens product. The product is formed by the process that includes sandwiching a polarizer between first and second layers of cellulosic material to form a polarizing insert. The process also includes shaping (e.g., thermoforming) the polarizing insert to have a curvature corresponding to the surface of a plate of an injection mold assembly. The process further includes injection molding a high-impact polymer adjacent at least one side of the insert by an injection/coining process. The process is performed to create minimal stress in the insert and the high-impact polymer.
In one aspect of the invention, a scratch-resistant coating is applied to at least one of the outer surfaces of the polarized lens.